Eastern Japan has hot and humid summers and cold winters with very heavy snow on the Sea of Japan side and in mountainous areas. Western Japan has very hot and humid summers (with temperatures sometimes reaching 35 oC or above) and moderate cold winters. Okinawa and Amami have a subtropical oceanic climate.
Tourists can go and swim especially the waters can be warm?
The Sea of Japan is decidedly cooler than the Pacific Ocean, but tends to be less developed. Air and water temperatures vary quite a bit from south to north, but swimming can be enjoyed in most places at least from mid July through August.
If you’ll be in this part of Japan between July and August, you should make a stop at Shimao Beach. One of Japan’s best swimming beaches, it’s located right on Toyama Bay, and is easily accessible from JR Shimao Station.